Java Lesson 2

1
Java Lesson 2

• Object Oriented Programming Concepts

2
OO Programming Concepts

• What is an Object?
• What is a Message?
• What is a Class?
• What is Inheritance?
• What is an Interface?
• A Closer Look at Hello World Application

3
What Is an Object?

• Definition An object is a software bundle of
  variables and related methods.
• Examples of objects dogs, chairs, tables, trees,
  bicycles, etc.
• For example
• Dogs have state (name, color, breed, hungry) and
  behavior (barking, fetching, and wagging tail).
• Bicycles have state (current gear, current pedal
  cadence, two wheels, number of gears) and
  behavior (braking, accelerating, slowing down,
  changing gears).

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What Is an Object?

• Software objects have state and behavior.
• A software object maintains its state in one or
  more variables. A variable is an item of data
  named by an identifier.
• A software object implements its behavior with
  methods. A method is a function (subroutine)
  associated with an object.
• You can represent real-world objects by using
  software objects.

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What Is an Object?

• Bicycle Model

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What Is an Object?

• Advantages
• Modularity The source code for an object can be
  written and maintained independently of the
  source code for other objects
• Information hiding An object has a public
  interface that other objects can use to
  communicate with it.

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What is a message?

• An object usually appears as a component of a
  larger program or application that contains many
  other objects.
• Software objects interact and communicate with
  each other by sending messages to each other.
• When object A wants object B to perform one of
  B's methods, object A sends a message to object B

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What is a message?

• The object to which the message is addressed
  (YourBicycle)
• The name of the method to perform (changeGears)
• Any parameters needed by the method (lowerGear)

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What is a message?

• Messages provide two important benefits.
• An object's behavior is expressed through its
  methods, so (aside from direct variable access)
  message passing supports all possible
  interactions between objects.
• Objects don't need to be in the same process or
  even on the same machine to send and receive
  messages back and forth to each other.

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What Is a Class?

• Definition A class is a blueprint, or prototype,
  that defines the variables and the methods common
  to all objects of a certain kind.
• Bicycles have some state (current gear, current
  cadence, two wheels) and behavior (change gears,
  brake) in common.

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What Is a Class?

• After you created the bicycle class, you can
  create any number of instances from that class.

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What Is a Class?

• Class Instance Variables
• Instance variables are in contrast to class
  variables (which are declared using the static
  modifier). The runtime system allocates class
  variables once per class regardless of the number
  of instances created of that class. The system
  allocates memory for class variables the first
  time it encounters the class. A class variable
  contains information that is shared by all
  instances of the class.
• All instances share a class variable. If one of
  them changes the value of a class variable then
  all other instances access this changed value.
• All instances share the same copy of the class's
  class variables. You can access class variables
  through an instance or through the class itself.

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What Is a Class?

• Bicycle class and its instance

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What Is Inheritance?

• Object-oriented systems allow classes to be
  defined in terms of other classes.
• For example, mountain bikes, racing bikes, and
  tandems are all kinds of bicycles.
• In object-oriented terminology, mountain bikes,
  racing bikes, and tandems are all subclasses of
  the bicycle class.
• Similarly, the bicycle class is the superclass of
  mountain bikes, racing bikes, and tandems.

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What Is Inheritance?
16
What Is Inheritance?

• Each subclass inherits state (in the form of
  variable declarations) from the superclass.
  Mountain bikes, racing bikes, and tandems share
  some states cadence, speed, and the like.
• Also, each subclass inherits methods from the
  superclass. Mountain bikes, racing bikes, and
  tandems share some behaviors braking and
  changing pedaling speed, for example.

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What Is Inheritance?

• Subclasses can add variables and methods to the
  ones they inherit from the superclass.
• Tandem bicycles have two seats and two sets of
  handle bars some mountain bikes have an extra
  set of gears with a lower gear ratio.
• Subclasses can also override inherited methods
  and provide specialized implementations for those
  methods.
• You are not limited to just one layer of
  inheritance. The inheritance tree, or class
  hierarchy, can be as deep as needed.

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What Is Inheritance?

• Inheritance offers the following benefits
• Subclasses provide specialized behaviors from the
  basis of common elements provided by the
  superclass. Through the use of inheritance,
  programmers can reuse the code in the superclass
  many times.
• Programmers can implement superclasses called
  abstract classes that define "generic" behaviors.
  The abstract superclass defines and may partially
  implement the behavior, but much of the class is
  undefined and unimplemented. Other programmers
  fill in the details with specialized subclasses.

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What is an Interface?

• An interface is a device that unrelated objects
  use to interact with each other
• Interfaces are useful for the following
• Capturing similarities among unrelated classes
  without artificially forcing a class
  relationship.
• Declaring methods that one or more classes are
  expected to implement.
• Revealing an object's programming interface
  without revealing its class.

20
A Closer Look At Hello World

• /
• The HelloWorldApp class implements an
  application that
• simply displays "Hello World!" to the standard
  output.
• /
• class HelloWorldApp
• public static void main(String args)
• System.out.println("Hello World!")
  //Display the string.

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Comments

• The Java language supports three kinds of
  comments
• / text /
• The compiler ignores everything from / to /.
• / documentation /
• This indicates a documentation comment (doc
  comment, for short). The compiler ignores this
  kind of comment, just like it ignores comments
  that use / and /. The JDK javadoc tool uses doc
  comments when preparing automatically generated
  documentation. For more information on javadoc,
  see the Java tool documentation .
• // text
• The compiler ignores everything from // to the
  end of the line.

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Classes

• In the Java language, each method (function) and
  variable exists within a class or an object (an
  instance of a class).
• The Java language does not support global
  functions or variables.
• Thus, the skeleton of any Java program is a class
  definition.

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The Main Method

• The entry point of every Java application is its
  main method.
• Every Java application must contain a main method
  whose signature looks like this
• public static void main(String args)
• The method signature for the main method contains
  three modifiers
• public indicates that the main method can be
  called by any object.
• static indicates that the main method is a class
  method.
• void indicates that the main method doesn't
  return any value.

24
Using Classes and Objects

• The other components of a Java application are
  the supporting objects, classes, methods, and
  Java language statements that you write to
  implement the application
• System.out.println("Hello World!")

25
Rectangle Example

• http//cse.yeditepe.edu.tr/taytekin/cse252/lab/Re
  ctangleApp1.java
• http//cse.yeditepe.edu.tr/taytekin/cse252/lab/Re
  ctangleApp2.java